It's the classic radio stutter. Normal background music plays perfectly, radio DJ & music plays perfectly if I switch on an external radio in the game world.
But playing it on my Pipboy causes the music to stutter at a certain regular intervals, but it never affects the DJ talking.

I've checked FFDShow and it's not this, fallout3.exe is already in the "Don't use ffdshow" list.

The next thing I tried was disabling the Fraunhofer codec and installing the MadFilter 1.0.0.3 one. At first this worked like a charm, but then the music suddenly glitched and plays at triple speed. Again, spoken voice of the DJs is unaffected, but the music plays as if Disney's Chipmunks.

What other solutions are out there, these are the only two I could find.